There's been a lot of talk here lately about "downsizing" boot sizes, and people complaining about how ski boots are supposedly all one size down from shoe size. In some ways I think it's a load of bs - you should size based on what fits you, not shoe size - 1 or something silly.
When I get ski boots sized, I don't work from what size shoes I wear, I'll start with what the manufacturer's sizing device says, and go from there, see how it shell fits, etc.
Neither shoe or boot sizes are an exact science or measurement. I can be one size in an a Adidas, another in Nike, and yet another in New Balance. I've seen posts on here complaining about how manufacturers are supposedly decieving with their sizing, when it's a fairly arbitrary scale to begin with.
Simple rule of thumb: get properly fitted in a competent shop. Then you know what size you are in that boot and manufacturer. In another boot, shoe, whatever, you will likely be a slightly different size. Feet are different, shoes and boots are different. Get used to it.